I don't see anyone else answering this, so maybe I'll take a crack at it.
Just to make sure, we are talking about the same thing, this half square
is really half of a bob-tail-curtain, right? (1/4 wave vertical,
connected at top by 1/2 wave wire, to another 1/4 wave vertical) and the
whole thing 10 ft off ground.
I don't understand how you are feeding this without a ground. This
antenna operates effectively as two verticals fed almost out of phase.
At the feed point, impedance wise, it looks like feeding an upside down
vertical. Very high input impedance. Since it is basically a vertical,
all antenna currents will be returning through the ground.
You are going to have to have a ground plane for this to work against.
Since this antenna is 10 ft in the air, you can't put the ground plane
on the ground at the feed point without making one of the verticals 10
ft longer, which means it no longer will perform like a half square.
There are two problems to consider, ground losses, and feedline
decoupling.
Since you are voltage feeding this, antenna feedpoint impedance is not a
problem. So for that purpose the ground plane doesn't have to be
resonant. However, for decoupling purposes a non resonant ground IS
going to be a problem. The feedline has will have no low impedance
ground to allow you to decouple currents from the feedline. I haven't
seen many of these antennas, and I don't know if anyone has any
ingenious solutions to this problem, but if I were building it, I would
put several 1/4 wave elevated radials at the feedpoint. I would also
try to measure feedline common mode currents after I did that, because I
suspect there will still be a lot. Then I would start adding ferrite
beads and maybe ground rods along the feedling if further reduction is
needed.
As far as ground losses are concerned, I have no way of accurately
calculating this. If you have no radials under the end opposite the feed
end, you have a very high voltage line 10 feet above ground which will
produce significant losses, I just don't know how much. You could also
add a ground plane under this end. I would think that this ground plane
could be either elevated or in the ground.
Jerry, K4SAV
mike l dormann wrote:
>hark - all you antenna enthusiasts out there, i have just put up a 40m
>half square, voltage fed, with the bottom of the vertical wires about 10
>feet off the ground. i was thinking of either putting some radials out
>from the fed end, or i have a piece of 3 foot wide rabbit cage fencing (
>zinc coated wire , 1 inch square holes) about 20 feet long.
>
>any thoughts would be greatly appreciated
>
>mike we'd
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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